The present invention relates to a variable-wavelength light-emitting element and to a method of fabricating the same, and more particularly to a variable-wavelength light-emitting element which utilizes a magnetic moment within semiconductor iron silicide (xcex2-FeSi2) and to a method of fabricating the same.
The phenomenon of the absorption spectrum of a semiconductor varying due to the Zeeman effect upon application of a magnetic field has been well known, and a semiconductor optical-modulating element which utilizes the Zeeman effect has been proposed (see Japanese Patent Application Laid-Open (kvokal) No. 9-246669).
In such a semiconductor optical-modulating element, since application of a magnetic field to the active layer of the element must be maintained even after removal of an external magnetic field, the electrodes of the element are formed of a ferromagnetic metal.
The above-described structure for a semiconductor optical-modulating element in which application of a magnetic field to the active layer of the element is continued even after removal of an external magnetic field seems to enable realization of a variable-wavelength light-emitting element which operates upon injection of current. However, the above-described structure has the following drawbacks.
When the thickness of a barrier layer which separates the ferromagnetic metal electrode from the active layer exceeds 0.1 xcexcm, the strength of a magnetic field which is produced by means of a magnetic moment within the ferromagnetic metal electrode and which is applied to the active layer is diminished considerably. Therefore, the barrier layer must be relatively thin. However, when the barrier layer is thin, electrons and holes within the active layer are likely to be affected by the center of non-radiative recombination present at the interface between the ferromagnetic electrode and the barrier layer. Therefore, the electrons and holes within the active layer annihilate without generation of light.
An example of a direct gap semiconductor having a magnetic moment is GaMnAs. This semiconductor is formed from GaAs through replacement of a few % of Ga atoms with atoms of Mn, which is a transition metal having a magnetic moment, and has a magnetic moment attributable to Mn. However, due to the difference in atomic radius between Ga and Mn, GaMnAs is in a greatly distorted state, and therefore is not suitable for the active layer of a light-emitting element.
An object of the present invention is to solve the above-described problems and to provide a variable-wavelength light-emitting element which employs a direct gap semiconductor having a magnetic moment for a semiconductor layer serving as an active layer, so that the semiconductor has reduced crystal distortion and stable characteristics as the active layer of the light-emitting element, as well as a method of fabricating the variable-wavelength light-emitting element.
To achieve the above object, the present invention provides the following:
(1) A variable-wavelength light-emitting element comprising: an active layer formed of a direct gap semiconductor having a magnetic moment; and p-type and n-type Si layers sandwiching the active layer and forming a pn junction region, the p-type and n-type Si layers having a forbidden bandwidth greater than that of the direct gap semiconductor, wherein the active layer is present in the pn junction region.
(2) A variable-wavelength light-emitting element described in (1) above, wherein the direct gap semiconductor is a semiconductor silicide.
(3) A variable-wavelength light-emitting element described in (2) above, wherein the semiconductor silicide is xcex2-FeSi2.
(4) A method of fabricating a variable-wavelength light-emitting element comprising the steps of: (a) forming, as an active layer, a xcex2-FeSi2 epitaxial layer on a first-conductive-type Si substrate; (b) changing the structure of the xcex2-FeSi2 epitaxial layer into a form of aggregated island-shaped grains; (c) growing a non-doped Si layer through molecular beam epitaxy, while heating the Si substrate to thereby transform the island-shaped xcex2-FeSi2 grains to xcex2-FeSi2 spheres and bury the xcex2-FeSi2 spheres into a monocrystal of the non-doped Si layer; and (d) growing a second-conductive-type Si layer through Si molecular beam epitaxy such that xcex2-FeSi2 is buried within an Si-pn junction depletion layer.
(5) A method of fabricating a variable-wavelength light-emitting element described in (4) above, wherein the formation of a xcex2-FeSi2 epitaxial layer in step (a) is performed such that the first-conductive-type Si substrate is heated to 470xc2x0 C. in an ultra-high vacuum, and Fe is deposited on the Si substrate to a thickness of 32 xc3x85 at a deposition rate of 0.1 xc3x85/s, so that the xcex2-FeSi2 epitaxial layer has a thickness about 3.2 times the thickness of the Fe deposition film.
(6) A method of fabricating a variable-wavelength light-emitting element described in (5) above, wherein a magnetic impurity is added in such an amount that the number of atoms of the magnetic impurity is one out of several tens to one out of 100 the number of atoms of Fe deposited during the Fe deposition.
(7) A method of fabricating a variable-wavelength light-emitting element described in (4) above, wherein the aggregation of island-shaped grains of the xcex2-FeSi2 epitaxial layer in step (b) is performed by means of annealing performed at 700 to 850xc2x0 C. for about one hour in an ultra-high vacuum.
(8) A method of fabricating a variable-wavelength light-emitting element described in (4) above, wherein the epitaxial formation of spheres of xcex2-FeSi2 in step (c) is performed such that non-doped Si is grown at a substrate temperature of 750xc2x0 C. such that the island-shaped xcex2-FeSi2 grains are further aggregated and transformed into spheres.
(9) A method of fabricating a variable-wavelength light-emitting element described in (4) above, wherein the formation of a second-conductive-type Si layer in step (d) is performed such that metaboric acid (HBO2) is deposited simultaneously with Si during the Si molecular beam epitaxy growth, and wherein the carrier density of the second-conductive-type Si layer can be controlled through regulation of the ratio in deposition rate between Si and HBO2.
(10) A method of fabricating a variable-wavelength light-emitting element described in (9) above, wherein when the deposition rate of Si is 0.4 xc3x85/s and the Knudsen cell temperature of metaboric acid (HBO2) is set to about 400xc2x0 C., the Si molecular beam epitaxial growth is performed such that the second-conductive-type Si layer has a carrier density of about 1018 cmxe2x88x923.
(11) A method of fabricating a variable-wavelength light-emitting element described in (4) above, wherein the buried xcex2-FeSi2 spheres have a diameter proportional to the thickness of the xcex2-FeSi2 epitaxial layer first grown.
As described above, incorporation of magnetic atoms into the semiconductor realizes a variable-wavelength light-emitting element in which electrons, photons, and magnetism act mutually.
For example, when an n-type Si substrate is heated to 470xc2x0 C. in an ultra-high vacuum, and Fe is deposited on the Si substrate to a thickness of 32 xc3x85, xcex2-FeSi2 film is formed through epitaxial growth. Through employment of a structure such that a layer of semiconductor silicide, which is a direct gap semiconductor having a magnetic moment, sandwiched between p-type and n-type Si layers having a greater forbidden bandwidth and forming a pn junction, there can be obtained a variable-wavelength light-emitting element in which, upon application of an external magnetic field, a magnetic moment within the silicide magnetizes the active layer to thereby change the wavelength of emitted light.